Re: [Xen-devel] [PATCHv7] x86/ept: defer the invalidation until the p2m lock is released

["Tian, Kevin" <kevin.tian@xxxxxxxxx>]

> From: David Vrabel [mailto:david.vrabel@xxxxxxxxxx]
> Sent: Tuesday, February 02, 2016 12:27 AM
> 
> Holding the p2m lock while calling ept_sync_domain() is very expensive
> since it does a on_selected_cpus() call.  IPIs on many socket machines
> can be very slows and on_selected_cpus() is serialized.

slows -> slow

> 
> It is safe to defer the invalidate until the p2m lock is released
> except for two cases:
> 
> 1. When freeing a page table page (since partial translations may be
>    cached).
> 2. When reclaiming a zero page as part of PoD.
> 
> For these cases, add p2m_tlb_flush_sync() calls which will immediately
> perform the invalidate before the page is freed or reclaimed.
> 
> Signed-off-by: David Vrabel <david.vrabel@xxxxxxxxxx>
[...]
> diff --git a/xen/arch/x86/mm/p2m-ept.c b/xen/arch/x86/mm/p2m-ept.c
> index c094320..43c7f1b 100644
> --- a/xen/arch/x86/mm/p2m-ept.c
> +++ b/xen/arch/x86/mm/p2m-ept.c
> @@ -263,6 +263,7 @@ static void ept_free_entry(struct p2m_domain *p2m, 
> ept_entry_t
> *ept_entry, int l
>          unmap_domain_page(epte);
>      }
> 
> +    p2m_tlb_flush_sync(p2m);
>      p2m_free_ptp(p2m, mfn_to_page(ept_entry->mfn));
>  }
> 
> @@ -1095,15 +1096,10 @@ static void __ept_sync_domain(void *info)
>       */
>  }
> 
> -void ept_sync_domain(struct p2m_domain *p2m)
> +static void ept_sync_domain_prepare(struct p2m_domain *p2m)
>  {
>      struct domain *d = p2m->domain;
>      struct ept_data *ept = &p2m->ept;
> -    /* Only if using EPT and this domain has some VCPUs to dirty. */
> -    if ( !paging_mode_hap(d) || !d->vcpu || !d->vcpu[0] )
> -        return;
> -
> -    ASSERT(local_irq_is_enabled());
> 
>      if ( nestedhvm_enabled(d) && !p2m_is_nestedp2m(p2m) )
>          p2m_flush_nestedp2m(d);

should we postpone nestedp2m flush similarly, which also incurs 
on_selected_cpus when holding p2m lock?

> @@ -1116,9 +1112,38 @@ void ept_sync_domain(struct p2m_domain *p2m)
>       *    of an EP4TA reuse is still needed.
>       */
>      cpumask_setall(ept->invalidate);
> +}
> +
> +static void ept_sync_domain_mask(struct p2m_domain *p2m, const cpumask_t 
> *mask)
> +{
> +    on_selected_cpus(mask, __ept_sync_domain, p2m, 1);
> +}
> +
> +void ept_sync_domain(struct p2m_domain *p2m)
> +{
> +    struct domain *d = p2m->domain;
> 
> -    on_selected_cpus(d->domain_dirty_cpumask,
> -                     __ept_sync_domain, p2m, 1);
> +    /* Only if using EPT and this domain has some VCPUs to dirty. */
> +    if ( !paging_mode_hap(d) || !d->vcpu || !d->vcpu[0] )
> +        return;
> +
> +    ept_sync_domain_prepare(p2m);
> +
> +    if ( p2m->defer_flush )
> +    {
> +        p2m->need_flush = 1;
> +        return;
> +    }
> +
> +    ept_sync_domain_mask(p2m, d->domain_dirty_cpumask);
> +}
> +
> +static void ept_flush_and_unlock(struct p2m_domain *p2m, bool_t unlock)
> +{
> +    p2m->need_flush = 0;
> +    if ( unlock )
> +        mm_write_unlock(&p2m->lock);
> +    ept_sync_domain_mask(p2m, p2m->domain->domain_dirty_cpumask);
>  }
> 
>  static void ept_enable_pml(struct p2m_domain *p2m)
> @@ -1169,6 +1194,7 @@ int ept_p2m_init(struct p2m_domain *p2m)
>      p2m->change_entry_type_range = ept_change_entry_type_range;
>      p2m->memory_type_changed = ept_memory_type_changed;
>      p2m->audit_p2m = NULL;
> +    p2m->flush_and_unlock = ept_flush_and_unlock;
> 
>      /* Set the memory type used when accessing EPT paging structures. */
>      ept->ept_mt = EPT_DEFAULT_MT;
> diff --git a/xen/arch/x86/mm/p2m-pod.c b/xen/arch/x86/mm/p2m-pod.c
> index ea16d3e..35835d1 100644
> --- a/xen/arch/x86/mm/p2m-pod.c
> +++ b/xen/arch/x86/mm/p2m-pod.c
> @@ -626,6 +626,7 @@ p2m_pod_decrease_reservation(struct domain *d,
> 
>              p2m_set_entry(p2m, gpfn + i, _mfn(INVALID_MFN), cur_order,
>                            p2m_invalid, p2m->default_access);
> +            p2m_tlb_flush_sync(p2m);
>              for ( j = 0; j < n; ++j )
>                  set_gpfn_from_mfn(mfn_x(mfn), INVALID_M2P_ENTRY);
>              p2m_pod_cache_add(p2m, page, cur_order);
> @@ -755,6 +756,7 @@ p2m_pod_zero_check_superpage(struct p2m_domain *p2m,
> unsigned long gfn)
>      /* Try to remove the page, restoring old mapping if it fails. */
>      p2m_set_entry(p2m, gfn, _mfn(INVALID_MFN), PAGE_ORDER_2M,
>                    p2m_populate_on_demand, p2m->default_access);
> +    p2m_tlb_flush_sync(p2m);
> 
>      /* Make none of the MFNs are used elsewhere... for example, mapped
>       * via the grant table interface, or by qemu.  Allow one refcount for
> @@ -886,6 +888,8 @@ p2m_pod_zero_check(struct p2m_domain *p2m, unsigned long
> *gfns, int count)
>          }
>      }
> 
> +    p2m_tlb_flush_sync(p2m);
> +
>      /* Now check each page for real */
>      for ( i=0; i < count; i++ )
>      {
> diff --git a/xen/arch/x86/mm/p2m.c b/xen/arch/x86/mm/p2m.c
> index a45ee35..36a8fb7 100644
> --- a/xen/arch/x86/mm/p2m.c
> +++ b/xen/arch/x86/mm/p2m.c
> @@ -325,6 +325,25 @@ void p2m_flush_hardware_cached_dirty(struct domain *d)
>      }
>  }
> 
> +/*
> + * Force a synchronous P2M TLB flush if a deferred flush is pending.
> + *
> + * Must be called with the p2m lock held.
> + */
> +void p2m_tlb_flush_sync(struct p2m_domain *p2m)
> +{
> +    if ( p2m->need_flush )
> +        p2m->flush_and_unlock(p2m, 0);
> +}
> +
> +void p2m_tlb_flush_and_unlock(struct p2m_domain *p2m)
> +{
> +    if ( p2m->need_flush )
> +        p2m->flush_and_unlock(p2m, 1);
> +    else
> +        mm_write_unlock(&p2m->lock);
> +}

prefer to move general stuff into this function, then you could just
keep a flush() callback, e.g.:

void p2m_tlb_flush_and_unlock(struct p2m_domain *p2m)
{
    if ( p2m->need_flush )
    {
        p2m->need_flush = 0;
        mm_write_unlock(&p2m->lock);
        p2m->flush(p2m);
    }
    else
        mm_write_unlock(&p2m->lock);
}

Same for p2m_tlb_flush_sync.

Thanks
Kevin


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